Gene Interaction - An Overview

Several investigations have revealed that a combination of various genes controls, influences and governs the majority of the characteristics of living beings.

Gene interaction is the phenomenon whereby a single character is controlled by two or more genes and each gene affects the expression of the other genes involved.

Gene interactions can comprise two or more pairs of genes. The phenotypic expression of the same character is affected by all the gene interactions involving two pairs of non-allelic genes. Modified dihybrid ratios are produced by these interactions.

Table of Contents

• What is Gene Interaction?
• Types of Gene Interaction
• Classification of Epistasis Gene Interaction
• Biological Significance
• Frequently Asked Questions (FAQs)

What is Gene Interaction?

Gene interaction is the process by which the expression of two or more genes influences one another in different ways as an organism develops a single characteristic. The majority of the traits that comprise living beings are coordinated by various genes.

Mendel and other researchers assumed that characters were controlled by a single gene, but it was later found that multiple characters were controlled by two or more genes. Such genes modify the conventional dihybrid (9:3:3:1) or trihybrid (27:9:9:9:3:3:3:1) ratios by influencing the development of the concerned traits in various ways.

The expression of one gene is influenced by the expression, present or absent, of another gene in a gene interaction.

Types of Gene Interaction

Gene interactions are divided into two categories:

1. Allelic or Non-epistatic Gene Interaction: This gene interaction occurs between the alleles of a single gene.
2. Non-allelic or Epistatic Gene Interaction: This gene interaction involves interactions between genes on identical or distinct chromosomes.

Allelic or Non-epistatic Gene Interaction

When phenotypic ratios diverge from Mendelian ratios, it is difficult for Mendelian genetics to explain some types of inheritance. This is because specific alleles can often be equally or partially dominant to each other or due to the lethal alleles. Allelic, non-epistatic or intra-allelic interactions are the terms used to describe genetic interactions between alleles of a single gene.

Incomplete Dominance (1:2:1)

A dominant allele could not entirely suppress the other allele. Thus, a heterozygote is phenotypically differentiated from either homozygote (intermediate phenotype).

In snapdragon and Mirabilis jalapa, the hybridisation of pure-bred red-flowered and white-flowered plants results in pink-flowered F₁ seedlings (deviated from parental phenotypes), which are intermediate between the two parents. When the F₁ plant self-fertilises, the F₂ progeny displays three classes of plants in the ratio of 1 red:2 pink:1 white rather than 3:1.

Codominance

Here, heterozygotes exhibit the expression of both alleles of a gene. Instead of the intermediate phenotype, the F₁ hybrid displays the phenotypes of both parents. In humans, a single gene regulates the MN blood group.

There are just two alleles: M and N. Children born to a father with the N blood type (genotype NN) and a mother with the M blood type (genotype MM) would have the MN blood type (genotype MN). The hybrid exhibits both phenotypes. Thus, the blood groups that F₂ segregates into are 1M:2MN:1N.

Over Dominance

F₁ heterozygotes occasionally have more extreme phenotypes than either of their parents. The heterozygous white eyes of Drosophila have more fluorescent eye pigment than either of its parents.

Lethal Factor

Lethal factors are genes that lead to the death of the person who has them. The heterozygotes are unaffected by recessive lethal because they only express when they are homozygous. Some genes dominantly affect phenotype yet are lethal when expressed in a recessive manner, such as the mouse gene for the yellow coat colour.

Multiple Alleles

There may be more than two allelomorphs or alleles at the same chromosomal location for a gene encoding a specific characteristic (only two are present in a diploid organism). These allelomorphs produce multiple alleles. The best example is the human ABO blood type.

Also, read: Co-Dominance and Multiple Alleles

Non-allelic or Epistatic Gene Interaction

When two or more genes affect each other’s expression in various ways, non-allelic or epistatic or inter-allelic interaction takes place, resulting in the development of a single character.

Simple Interaction (9:3:3:1)

Here, the same character is influenced by two non-allelic gene pairs. When any of the two factors is present independently, its dominant allele results in different phenotypes. When both dominant alleles are present, a distinctive phenotype is produced. Another phenotype results from the absence of both dominant alleles.

Complementary Factor (9:7)

The interaction of two or more genes inhabiting distinct loci acquired from different parents results in the development of specific characteristics. These genes are complementary to each other, which means they do not express themselves when present separately. Instead, they only express themselves when introduced together through a suitable crossing.

More than two complementary genes can be involved; for example, three complementary genes control the aleurone colour of maize.

Epistasis

Epistasis occurs when a gene or gene pair suppresses or hinders the expression of another non-allelic gene. The gene that causes the effect is referred to as an epistatic gene, while the gene whose expression is inhibited is referred to as a hypostatic gene.

Also, read: Difference between Homozygous and Heterozygous

Classification of Epistasis Gene Interaction

Epistatic gene interaction is categorised as follows based on how the involved genes affect one another’s expression:

• Supplementary gene interaction
• Complementary gene interaction
• Inhibitory gene interaction
• Duplicate gene interaction
• Masking gene interaction
• Polymeric gene interaction

Supplementary Gene Interaction

In supplemental gene interaction, the phenotypic effect is produced by the dominant allele of one of the two genes regulating a character. However, the dominant allele of another gene has no independent phenotypic effect.

Therefore, it changes the phenotypic effect brought on by the first gene when it coexists with the dominant allele. For example, Agouti (grey) coat development in mice.

F₂ Generation:

Thus, the phenotypic ratio is 9 (Agouti):3 (coloured):4 (Albino).

Complementary Gene Interaction

The F₂ ratio becomes 9:7 instead of 9:3:3:1 if both gene loci are homozygous and both produce the same phenotype. In this instance, a single phenotype is produced by the genotype aaBB, aaBb, Aabb, aabb.

When both dominant alleles are present simultaneously, they are referred to as complementary genes and result in a distinct phenotype.

In sweet pea, the homozygous presence of the genes CC, cc, PP, and PP results in no colour (white) as chromogen production is not possible in a homozygous state, but is possible in a heterozygous gene condition.

Inhibitory Gene Interaction

When the heterozygous (Bb) and homozygous (BB) forms of a dominant allele at a single gene locus (B) result in the same phenotype, the F₂ ratio changes from 9:3:3:1 to 13:3. A distinctive phenotype is produced by the homozygous recessive (bb) condition.

It is known as inhibitory gene interaction when a homozygous recessive (bb) trait inhibits the phenotypic expression of the other genes.

Duplicate Gene Interaction

When an identical phenotype is produced by the dominant allele of both gene loci without any cumulative impact, it is referred to as duplicate gene interaction. In this type of interaction, the ratio changes to 15:1 rather than 9:3:3:1. Shepherd’s purse plant exhibits duplicate gene interaction.

Plant seed capsules from the shepherd’s purse species can be either triangular or oval. When both the genes are present in a homozygous recessive state, the seed capsule adopts an ovoid shape.

Masking Gene Interaction

When the dominant allele (A) of one gene masks the activity of the allele of another gene (B), it is referred to as masking gene interaction. It is thus said that the locus of gene A is epistatic to the locus of gene B.

Dominant epistatic relationships are those in which the dominant allele A expresses itself only when B or b is present. Only when the epistatic locus allele is homozygous recessive does the hypostatic locus allele express itself.

Polymeric Gene Interaction

Two dominant alleles working together to intensify the phenotype or produce a median variance is known as polymeric gene interaction. Each dominant allele alone results in a physical characteristic which is distinct from the combination of dominant alleles. This results in three phenotypes being produced from just two dominant alleles. As a result, neither dominant allele is outperforming the other dominant allele.

Biological Significance

Genetic interactions have significant effects on how genotype and phenotype are related. They have been put out as an explanation for missing heritability. The term “missing heritability” describes how many heritable characteristics still have unidentified genetic ancestors.

Genetic interactions could significantly reduce the amount of missing heritability, despite the many hypotheses that have been presented. These genetic interactions would probably transcend the pairwise interactions taken into account in genetic interaction networks.

Therefore, the expression of genes is not independent of one another and depends on the presence or absence of other genes. This type of deviation from the Mendelian principle of one gene-one trait is termed the Factor Hypothesis or Gene Interaction.

Related Links:

• Mendel’s Laws of Inheritance
• Important Notes for NEET Biology – Principles of Inheritance and Variation
• Important Notes for NEET Biology – Molecular Basis of Inheritance

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